Liquid crystal displays (LCD) have been widely used in many different applications in recent years. Because of the light weight and small volume, most of notebook or lab-top computers use LCDs as their video monitors. An LCD manufactured with a thin-film transistor (TFT) array provides a video monitor with high solution, rich colors and fast response.
An LCD relies on the orientation of the liquid crystal positioned between the two glass substrates to pass or block light of each pixel on the display. In a twisted-nematic (TN) mode, the orientation of the liquid crystal is controlled by the voltage difference between the transparent upper and lower electrodes of each display pixel. By changing the voltages applied to the electrodes, the liquid crystals can be aligned differently for different pixels to provide various pixel intensities in an image.
LCD manufacturers have been trying to manufacture larger LCDs to replace the conventional cathode-ray tube (CRT) display monitors of desk-top computers. Due to the birefringence effect in a twisted-nematic LCD, the viewing angle is limited to a small range that is not acceptable in a larger display monitor.
In-plane switching (IPS) is one of more recent technologies that have been used in manufacturing TFT LCDs. In the in-plane switching technology, the parallel electric field applied to the comb-shaped electrodes formed on the lower glass substrate of the LCD orients the liquid crystals. As compared to a TN LCD, an IPS LCD has a wider viewing angle. 13.3" and 14.1" IPS LCDs with SVGA(800.times.RGBx600) or XGA(1024xRGBx768) resolution have been manufactured for the video monitors of desk-top computers.
FIG. 1 shows the conventional process of fabricating an IPS array in an LCD. In the conventional process, a glass substrate 101 is first prepared. A first layer of Cr, a layer of Mo and a second layer of Cr are coated on the glass substrate 101 by using a sputtering technique after it is cleaned. These metal layers form the M1 layer of the LCD. By patterning and wet-etching these layers, the gate 102, the scan line and the common line (not shown in the cross sectional view) are formed on the glass substrate 101.
An SiNx layer 105, an a-Si:H layer 106 and an n.sup.+ a-Si:H layer 107 are then deposited on top of the structure that has been formed on the glass substrate as shown in FIG. 1(B). An ITO layer 108 is coated on top of the LCD structure formed so far. Only the peripheral area that needs to be wired is patterned to form pads for attaching TABs. A contact hole is formed for connecting M1 and M2 layers.
A Cr layer and an aluminum layer that constitute the M2 layer are then coated. The source 109 and the drain 110 of the TFT as well as the data line (not shown in the cross-sectional view) of the LCD are patterned using photo-resist. After removing the photo-resist, the M2 layer is used as a hard mask for etching the n.sup.+ a-Si:H layer through back-channel etching.
A SiNx layer is deposited as a passivation layer. On the passivation layer, only the peripheral area for wiring is opened and the pixel area remains covered. The IPS device is thus formed as shown in FIG. 1(C).
One of the problems in the conventional process is related to the thickness of the M2 layer. It has been observed that on the edge of the M2 layer electrodes, light leaks significantly after the LCD is packaged. The M2 layer generally comprises 500 angstroms thick of Cr and 6000 angstroms thick of Al. Because the electrode is formed with the thick M2 layer, near the electrode edge back to the rubbing direction, the area can not be well rubbed in a rubbing process.
The insufficient rubbing results in the fact that the liquid crystal can not be oriented appropriately during the normal operation of the LCD. Consequently, it causes light leak. The contrast of the LCD is thus greatly decreased and the quality of the display is degraded.
Although the light leak problem may be reduced by decreasing the thickness of the M2 layer, it is found that the light leak can not be completely eliminated. In addition, decreasing M2 layer's thickness increases the resistance of the data line that slows the response of the LCD.